(1) Field of the Invention
This invention relates to an X-ray tube for use with a CT (Computerized Tomography) apparatus, which is capable of a high-speed scan of X-ray emitting positions circumferentially of an examinee.
(2) Description of the Related Art
The X-ray CT apparatus is used to obtain images of X-ray absorptivity distribution in cross sections of an examinee by emitting X-rays from varied directions through 360 degrees (or 180 degrees) around the examinee and putting the multi-directional X-ray transmission data thereby collected to image regeneration processing. In order to collect multi-directional X-ray transmission data, the X-ray CT apparatus usually has an X-ray tube rotatable by a rotating mechanism to emit X-rays from varied directions around an examinee.
With the rotation of the X-ray tube itself, however, data cannot be collected quickly since it takes about one second for the X-ray tube to make one complete rotation or a half rotation to obtain a single slice image. The above scanning method is not fit for examination of an organ such as the heart whose movement can be grasped only with high-speed imaging in the order of 30 frames per second.
In view of the above drawback, an X-ray tube for the CT apparatus has been proposed in recent years, which is capable of running an X-ray generating position on a circumference at a very high speed. The known X-ray tube for the CT apparatus will be described hereunder with reference to FIG. 1. This X-ray tube comprises a bell-shaped vacuum tube 1, and an electron gun 2 connected to a proximal end of the vacuum tube 1. The vacuum tube 1 contains deflecting coils 3, deflecting electrodes 4, and a ring-shaped target 5. An electron beam 6 emitted from the electron gun 2 is deflected by the deflecting coils 3 and deflecting electrodes 4 to impinge on the target 5. As a result, an X-ray 7 is emitted from the target 5 toward a central part of the vacuum tube 1. By controlling the deflecting coils 3 and deflecting electrodes 4, an X-ray generating position (focal point) 8 is caused to run at high speed along the circumferential wall of the target 5. Consequently, the X-ray 7 is emitted from varied directions around an examinee M introduced into the central part of the vacuum tube 1. In this way, a picture for one frame is picked up in about 50 msec, for example.
With the known X-ray tube for the CT apparatus, however, the electron beam 6 is run in the direction perpendicular to a plane formed by the ring-shaped target 5 or by the circumference on which the X-ray generating position 8 moves, and the electron beam 6 is deflected in the course of its run. Consequently, the X-ray tube must have a very large construction about 4 meters long in the direction perpendicular to the plane formed by the ring-shaped target 5 (i.e. axially of the examinee M). An X-ray CT apparatus using such an X-ray tube requires a large installation space.